Thyroid & parathyroid

Congenital / metabolic anomalies

Parasitic nodule

Topic Completed: 1 December 2015

Minor changes: 27 January 2021

Copyright: 2003-2021,, Inc.

PubMed Search: Parasitic nodule [title]

Andrey Bychkov, M.D., Ph.D.
Page views in 2020: 745
Page views in 2021 to date: 768
Cite this page: Bychkov A. Parasitic nodule. website. Accessed November 29th, 2021.
Definition / general
  • Also called sequestered (i.e. sequestered goiter), detached or accessory thyroid nodule
  • Recommended to use "parasitic nodule" for separated thyroid nodules in lateral neck, as opposed to midline ectopic thyroid tissue along the thyrothymic tract, which is mainly a developmental abnormality (Virchows Arch 1999;434:241)
  • Lateral aberrant thyroid often represents parasitic thyroid nodule
  • F:M = 4:1, median age is 51 years (range 15 to 83 years)
  • ~100 cases have been reported; the largest series was from Dr. Rosai (Lab Invest 2006;86:96A)
  • Perithyroidal, close to the gland (< 1 cm)
  • Can be located in the lateral neck from the submandibular to the retroclavicular area, the sternocleidomastoid and sternohyoid muscles (Lab Invest 2006;86:96A)
  • Rarely found in the mediastinum as part of a substernal nodular goiter (Arch Intern Med 1983;143:1015)
Pathophysiology / etiology
  • Portion of goitrous thyroid extending through the fascia may be separated by the mechanical action of neck muscles, and remains connected to the main gland by a thin fibrous strand of vascular tissue (Boston Med Surg J 1903;149:616)
  • Split from thyroid gland is due to ablation of pre-existing connection or lack of identification of connection to the main gland (Wenig: Atlas of Head and Neck Pathology, 3rd Edition, 2015)
  • Alternatively, parasitic nodule may represent concurrent hyperplastic changes in accessory thyroid tissue (N Engl J Med 1964;270:927)
  • Blood supply may be obtained from thyroid via fibrovascular pedicle, or be autonomous, acquired from the surrounding tissues (ISRN Surg 2011;2011:313626)
Clinical features
  • Palpated in the lateral neck (N Engl J Med 1964;270:927)
  • The nodule is usually an expression of nodular hyperplasia or nodular Hashimoto thyroiditis, less commonly of Graves disease (Histopathology 2006;49:107)
  • Benign condition. but some cases of metastatic thyroid carcinoma from occult primary may be initially misdiagnosed as parasitic nodules
    • Rodriguez found malignancy without evidence of tumor in the main gland in 10% of studied parasitic nodules, and suggested that parasitic nodule can originate in a primary tumor (Lab Invest 2006;86:96A), but microcarcinoma in the main thyroid cannot be excluded
  • On histopathology, after exclusion of metastatic cancer
Radiology description
Radiology images

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Neck ultrasonography

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Color flow Doppler

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Tracer uptake, lateral neck

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Radiography of mediastinal mass

Case reports
  • Usually removed surgically to rule out metastasis
Gross description
  • 0.5 - 6.5 cm nodule, separate from thyroid gland, usually single (> 80%)
  • Fibrovascular pedicle connecting to the main thyroid can be discovered after careful dissection at surgery
  • Often nodular or shows changes similar to the main thyroid
Gross images

AFIP images
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Nodular hyperplasia

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Mediastinal thyroid mass

Microscopic (histologic) description
  • Benign appearing thyroid tissue with colloid filled or hyperplastic follicles
  • Similar features are found in orthotopic gland
  • Hashimoto thyroiditis in parasitic nodule may simulate lymph node tissue
Microscopic (histologic) images

Contributed by Andrey Bychkov, M.D., Ph.D.

Two perithyroidal lymph nodes

Specimen with signs of Hashimoto thyroiditis

Lymphoid follicles

AFIP images
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Patient with Hashimoto

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Hyperplastic nodule

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Parasitic nodule vs lymph node

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Thyroid follicles

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TTF1, Thyroglobulin

Cytology description
Negative stains
Molecular / cytogenetics description
Differential diagnosis
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